As energy source prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future life. Thus, research into various power generation techniques such as nuclear energy, solar energy, wind energy, tidal power, and the like, is underway, and power storage devices for more efficient use of the generated energy are also drawing much attention.
In particular, demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and demand therefor continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized and the market for lithium secondary batteries continues to expand to applications such as auxiliary power supplies through smart-grid technology.
In general, lithium secondary batteries has a structure wherein an electrode assembly, composed of a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material and a porous separator, is impregnated with a lithium electrolyte. A positive electrode is manufactured by coating a positive electrode mix including a positive electrode active material on an aluminum foil, and a negative electrode is manufactured by coating a negative electrode mix including a negative electrode active material on copper foil.
In such lithium secondary batteries, carbon-based materials are mainly used as a negative electrode active material, and use of lithium metals, sulfur compounds, silicon compounds, tin compounds, etc. is also considered. In addition, lithium-containing cobalt oxides such as LiCoO2 are mainly used as a positive electrode active material. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
LiCoO2 is widely used due to excellent overall physical properties such as excellent cycle properties, but low in safety. In addition, due to resource limitation of cobalt as a raw material, LiCoO2 is expensive and massive use thereof as a power source in fields such as electric vehicles and the like is limited. Due to characteristics of preparation methods of LiNiO2, it is difficult to apply LiNiO2 to mass production at reasonable expenses. In addition, lithium manganese oxides such as LiMnO2 and LiMn2O4 have disadvantages such as poor cycle characteristics.
Accordingly, a method of using a lithium transition metal phosphate as a positive electrode active material is under study. The lithium transition metal phosphate is broadly classified into LixM2(PO4)3 having a NaSICON structure and LiMPO4 having an olivine structure, and considered as a material having superior stability, when compared with existing LiCoO2. At present, Li3V2(PO4)3 having a NaSICON structure is known, and LiFePO4 and Li(Mn, Fe)PO4 among compounds having an olivine structure are most broadly studied.
Among compounds having the olivine structure, LiFePO4 has a voltage of 3.5 V and a high bulk density of 3.6 g/cm3 with respect to lithium, and a theoretical capacity of 170 mAh/g. In addition, LiFePO4 has superior high-temperature stability, compared to cobalt (Co), and uses cheap Fe as a raw material. Accordingly, applicability of LiFePO4 as a positive electrode active material for lithium secondary batteries is high.
However, such LiFePO4 has problems as follows, and thus, commercialization thereof is limited.
First, internal resistance of a battery increases due to low electron conductivity of LiFePO4 when LiFePO4 is used as a positive electrode active material. Accordingly, polarized potential increases when battery circuits are closed, thereby resulting in reduction of battery capacity.
Second, since LiFePO4 has lower density than general positive electrode active materials, it is limited to sufficiently increase energy density of a battery.
Third, since an olivine crystal structure in which lithium is desorbed is very unstable, a migration pathway of a crystal surface in which lithium is desorbed is blocked, whereby absorption/desorption rates of lithium are decreased.
Accordingly, technology to shorten migration distances of lithium ions by reducing a crystal size of olivine to a nanoscale, thus increasing a discharge capacity, has been suggested.
However, when an electrode is manufactured using such olivine particles having minute diameter sizes, a large amount of binder should be used and time of mixing a slurry is extended, thereby decreasing process efficiency. When secondary particles in which primary nanoparticles are physically agglomerated are used to address such problems, processability is enhanced, but battery capacity characteristics and output characteristics may be deteriorated due to the diffusion rate of lithium cations in the secondary particles which is slower than that in primary particles.
Therefore, there is an urgent need for technology to provide high process efficiency while providing superior battery performance.